The world is fast running out of fossil fuels and with an energy crisis looming, intensive research is being carried out across the globe to find renewable alternatives. Top of the list are biofuels; fuels derived from biomass. Will the plants grown to provide this biomass behave themselves when introduced to sites outside their native range, or escape cultivation and invade the regions to which they are introduced?

There are various classes of biofuel, all with their pros and cons and all of which are likely to have a role in fuelling the future. The so-called first generation biofuels come from either sugar-rich plants (e.g. sugar cane) or starchy plants such as wheat and maize, the starch from which is hydrolysed to sugars which are then fermented to produce alcohol-based fuels. First generation biofuels are in major production worldwide, however with many feedstocks having primary roles as food crops, their use for fuel has led to widespread concern at the risk of inflating food prices, along with other issues such as increasing levels of deforestation to allow for expansion of cropping. First generation biofuels also include feedstock species such as Jatropha curcas, which bear seeds that can be pressed to release a hydrocarbon rich oil, which is processed to give ‘biodiesel’. Third generation biofuels, particularly those that enlist algae to produce biodiesel, are seen by some as the ‘future’ of biofuels. Biodiesel production using algae has been demonstrated experimentally, but the successful large scale production and commercial viability of third generation biofuels are yet to be proven.

It is the second generation biofuels that are being relied upon to make up the major component of biofuel targets set by leading Governments such as that of the USA. Second Generation biofuels are those produced from cellulosic biomass, generally with woody or fibrous feedstocks such as Miscanthus, Arundo donax (giant reed), and Panicum virgatum (switchgrass). Crop residues usually left in the field may also be used as a feedstock for second generation biofuels. Though processing technologies are advancing, particularly with regard to the difficult task of breaking down the tough lignocellulosic structure of second generation biofuel feedstocks, an increasing number of concerns have been voiced in relation to carbon emissions resulting from indirect land use change (ILUC) for biofuel production and the the nature of the feedstock plants themselves.

An ideal second generation biofuel feedstock would have certain key qualities. For example, the plant would: be fast growing; have few pests and diseases or be resistant to any present; be able to use water efficiently; be propagated vegetatively; be able to outcompete undesirable species; and have efficient photosynthetic systems. Many of the species selected as Second Generation biofuels have some or all of these traits. It is now, however, that alarm bells start to ring – these traits are also common to a far less welcome selection of plants, namely invasive weeds. This fact has been brought to light by a number of esteemed ecologists (for example Raghu et al., 2006; Low & Booth, 2007; DiTomaso et al., 2007; Simberloff, 2008) and has been taken on board by some, leading to proposed biofuel-specific weed risk assessments (WRAs) being produced for certain regions such as Europe and Hawaii (Crosti et al., 2009; Buddenhagen et al., 2009). Whilst WRAs may prove to be effective once integrated into a country’s import legislation, many regions will lack stringent controls over the introduction of new plant species, at least in the near future. This is likely to be particularly apparent when the plant to be introduced is proposed as a source of green fuel, particularly in less developed countries that may be targeted by major investors for their climatic suitability and availability of cheap land and labour where these potential sources of revenue may be snapped up and planted before opponents have a chance to protest. The combination of introducing plants to new regions (natural enemy escape), planting them en mass in multiple locations (propagule pressure) and the inherently weedy nature of the plants themselves may just lead to an array of new invasions.

The suspected weediness of biofuel feedstock species is not based on paranoid theory. A number of species both proposed and accepted as biofuel feedstocks have weedy track records. For example, Arundo donax (giant reed – see image, above), has become invasive outside its native range, for example in coastal southern California where it has become the principal threat to riparian ecosystems following widespread plantation (high propagule pressure) in the 1950s to prevent erosion. The plant is listed on the ISSG/IUCN 100 of the World’s Worst Invasive Alien Species list. Phalaris arundinacea (reed canary grass) has a similarly shady history, invading wetlands, displacing native species and forming monoculture stands. Despite this it is proposed as a potential feedstock for temperate regions because of its ability to thrive in moist, cool environments. A number of Prosopis (mesquite) species are also being suggested as biofuel feedstocks due to their rapid growth and limited nutrient requirements. This is despite the fact that many of these species and their hybrids have become invasive in Australia, Asia and Africa and are now the target of biocontrol programmes in South Africa and Australia.

What would make future biofuel feedstock invasions very hard to accept is that the risk has been widely forseen and reported, with various mitigating steps proposed. Will multinational energy investors after a quick buck hold more sway than concerned ecologists? Will the fuel feedstocks of the future run rampant and cause havoc or sit politely in their designated plots? Only time will tell…